1. Field of the Invention
The present invention relates to a bar coater, and a bar coating method. More specifically, the presently disclosed subject matter relates to a coating technique for producing an optical film having suitable quality for use in liquid crystal display devices.
2. Description of the Related Art
In a coating process involving the production of an optically functional film such as an optical compensation film, it is required to dispense a coating liquid uniformly to form a thin layer. Such thin layer coating, however, often causes longitudinal stripes and streak defects. Therefore, various measures have been conventionally studied and taken to prevent such defects.
For example, in order to suppress the occurrence of an irregular vortex in a reservoir located at the primary side (the upstream side of a web traveling direction) of a coater, there has been proposed to use the length of 10 mm to 50 mm for the reservoir in the web traveling direction (Japanese Patent Application Laid-Open No. 2003-33702).
In bar coaters, it is proposed to prevent stepped nonuniformity of coating due to vibration of a bar, and coating streak defects due to a vortex generated between the bar and a bar support member, by defining a relationship between a maximum radius of a cross section of the bar and a curvature radius of a circular arc in cross section of a bar receiver, a bar-holding angle, and the like (Japanese Patent Application Laid-Open No. 2006-82059).
There has been also proposed a bar coating method in which a wrap angle of the web with respect to the bar is set from 2.5° to 30°; a ratio of Rb/Rh is maintained in the range of from 0.9 to 1.0, where Rb designates a maximum radius of the bar cross-section, and Rh designates a curvature radius of a circular arc in cross section of a bar receiver serving as a bar support member; and a bar-holding angle of the bar support member is set to be 90° or more and 180° or less (Japanese Patent Application Laid-Open No. 9-201563).
Various methods proposed include a method in which a shearing force is applied to a coating liquid before it squirts out of a fountain nozzle in a blade coater, so as to reduce the viscosity of the coating liquid (Japanese Patent Application Laid-Open No. 7-189196), and a method of providing on a rod-bar holder a foreign matter-removing brush for removing foreign matter and aggregates caught in a gap between rod wires (Japanese Patent Application Laid-Open No. 7-246358). Also, in a coating method, in which after a coating film is formed on a substrate, and excess liquid is scraped off by a scraping roller to obtain a necessary film thickness, in order to prevent coating defects caused by dryness of the surface of the scraping roller, a method is proposed to supply a surface of the scraping roller with a coating liquid or solvent (Japanese Patent Application Laid-Open No. 9-294956).